1. Field of the Invention
This invention relates generally to composite materials formed from powder, and more particularly to silicon nitride-silicon carbide composite materials formed from powder and which exhibit excellent strength as well as high density and thermal conductivity.
2. Description of the Prior Art
Both silicon carbide (SiC) and silicon nitride (Si.sub.3 N.sub.4) are ceramic materials which have been seriously considered and tested for gas-turbine engine components because of their ability to withstand thermal shock conditions inherent to gas-turbine operations. Each of these materials possesses certain advantages over the other. For example, silicon nitride has lower thermal expansion and higher fracture toughness than silicon carbide. However, silicon carbide has a higher thermal conductivity than silicon nitride which is a very desirable feature for the components of gas-turbine engines. Thus, in selecting the material between silicon carbide and silicon nitride the selection of either material necessitated sacrificing the advantages of the other material in certain areas.
The density of silicon nitride and to a certain extent the strength of the composite material in which silicon nitride is the matrix component is dependent upon the manner in which the same is produced. N. G. Parr et al in "The Technology and Engineering Applications of Reaction-Bonded SILICON NITRIDE" J.R.N.S.S. Vol. 22, No. 3, pages 133-141 describes the reaction-bonding (sintering) of silicon nitride and reports at page 137 both the density and porosity which indicates the highest density they attained is only about 85 percent of the theoretical. This same paper deals with the effect of density and strength and indicates higher densities exhibit better strengths. Since better strengths are obtained where the silicon nitride approaches theoretical density and since silicon carbide is inherently weaker than silicon nitride, the improved thermal conductivity associated with the silicon carbide component also acts to diminish the strength of the composite material. Consequently, it is incumbent to obtain the highest density in the matrix material for improved strength.
One manner in which this was attempted was to partially reaction sinter silicon powder in a nitrogen-bearing atmosphere and thereafter raise the temperature in such a manner as not to form a molten pool of silicon metal and then the balance of the unreacted silicon was converted to silicon dioxide by appropriately changing the atmosphere. This process, as described by Parr in U.S. Pat. No. 3,215,547 was effective for improving the strength although no data are set forth for comparative purposes. Parr also disclosed a ternary composition of silicon nitride, silicon dioxide and up to 10 percent silicon carbide. This latter component is described as improving the creep strength of the composite material. In contrast, the function of the silicon carbide in the present invention is to improve the thermal conductivity. Concurrently therewith, slight decreases in room temperature flexural strength and creep strength exhibited by the composite material are noted in comparison with a composition comprising substantially all silicon nitride. These competing factors must be balanced in the composition of the present invention.